The invention relates to processes and apparatuses for separation of gaseous components by means of selective and adiabatic adsorption and desorption of usually considered unwanted impurities, using suitable adsorbents. The processes concerned are commonly known under the name pressure swing adsorption. Many different systems have been described in the patent literature, all characterized by the general feature that removal of at least one impurifying component is effected through selective adsorption at high pressure by at least one type of adsorbent, densely packed in a pressure vessel, called adsorber. During the adsorption step, feed gas is introduced at the inlet end of the adsorber, producing what is called primary product at the outlet end thereof. Dependent on the feed gas composition, adsorbers may contain more than one type of adsorbent, packed in different vertical zones on top of one another. Types of adsorbents, commonly used in the art may include zeolitic molecular sieves, activated carbon, silica gel or activated alumina.
The indications cocurrent and countercurrent, used hereafter in the description of the various process steps are related to the direction of feed gas flow inside the adsorbers during the adsorption step.
The adsorbents are regenerated by desorbing components through counter-current depressurization until the lowest pressure, producing what is called dump gas and by countercurrent purging at the lowest pressure with near product quality gas, producing purge offgas.
After adsorption, additional near product quality gas of lower pressure is recovered, hereafter called secondary product gas, by depressurizing the adsorber co-currently with the feed inlet end closed. This gas originates from the total void space in the adsorber and from fractionated desorption from adsorbents therein. Production of said secondary product gas by said cocurrent depressurization is made possible without a significant breakthrough of an adsorption front of an unwanted component, provided the adsorption step is ended early enough.
Final depressurization takes place countercurrently down to the lowest pressure, thereby releasing said dump gas. Such dump gas consists of at least one desorbed component in admixture with some of the product component.
The purging of the adsorbent takes place at the preferably lowest pressure by a countercurrent flow through it of purge gas, being a part of said secondary product gas, which thereafter, enriched with at least one desorbed component is collected as said purge offgas.
The combined streams of dump gas and purge offgas are discarded as offgas. Repressurization of the adsorber is realized with its inlet end closed, by admission through its outlet end of, (1) the remaining part of said secondary product gas for the hereinafter as such indicated low level repressurization and finally (2) a part the high pressure product gas, available as a split-off thereof, for the hereinafter as such indicated high level repressurization. On reaching the highest pressure, the contained regenerated adsorbents are ready to undergo a new adsorption step.
According to U.S. Pat. No. 3,430,418 to J. L. Wagner, a minimum of four adsorbers is required for a continuous operation, without requiring additional gas storage vessels, such that always at least one of them is used for adsorbing impurities from feed gas, while the adsorbents in the remaining adsorber or adsorbers are undergoing the other process steps of cocurrent and/or countercurrent depressurization, purging and repressurization.
For a given set of process conditions of feed gas composition, feed gas pressure and desirable offgas pressure, while aiming for a maximum product recovery efficiency, a certain optimum can be established with respect to the distribution of the secondary product gas over its reuse for purging and for low level repressurization. Maximum product recovery efficiency is consistent with the lowest possible concentration of the product component in the offgas, a condition which is met by using a restricted amount of purge gas. If the restricted amount of purge gas is sufficient for an adequate degree of adsorbent regeneration, any amount of available secondary product gas in excess of said restricted amount of purge gas should be used in a useful manner. It is one of the subjects of this invention to improve the utilization of said excess for low level adsorber repressurizations.
The effect of using more or less purge gas as clarified by the diagram of FIG. 1, showing the molar concentration of product component in the offgas each adsorber produces during dumping and purging. The value xe2x80x9cCxe2x80x9d of said concentration is plotted versus the percentage xe2x80x9cQxe2x80x9d of the total of such offgas. The composition of the front end of the dump gas, shown at the left hand side of said diagram, is always identical to the composition of the feed gas; as it originates from the void spaces of the inlet end of an adsorber and its connected piping. While dumping continues towards a continuously dropping pressure, the adsorbent""s void space releases gas which due to desorption contains more and more impurities, causing the value C to drop. Upon reaching the purging pressure, dumping is stopped. At this point Qd percent of the total offgas which an adsorber releases during a cycle has been produced. The amount indicated between Qd and 100% represents the purge offgas, the concentration of product component therein gradually rises until the available quantity of purge gas has been spent and the purging is finished. This clarifies, that by using less purge gas, less purge offgas is produced, containing less product component as well, consistently resulting in a higher product recovery efficiency. However, a sufficient quantity of purge gas remains required for an effective regeneration of the adsorbents near the outlet end of the adsorbers and to realize a sufficiently large loading difference after the adsorption step consistent with a commercially acceptable utilization of the adsorbents.
By increasing the internal pressure recovery efficiency, defined as pressure rise realized by secondary product gas relative to the pressure rise with total repressurization, then a smaller complementary part of said secondary product gas can be used for purging. One way of achieving a better control over the recovery and the distribution of secondary product gas over low level repressurization and purging is described in U.S. Pat. No. 3,564,816 granted to L. B. Batta. According to this description for a system consisting of 4 adsorbers, the internal pressure recovery efficiency is increased and the proportion of the total recovered quantity of secondary product gas reused for purging is reduced effectively with respect to realizing a higher product recovery efficiency. Increase of the internal pressure recovery efficiency in this system is realized by using the tail end of the released secondary product gas for the initial low level repressurization of an adsorber instead of for continuation of the purging of said adsorber.
Although this method, hereafter indicated as Batta-method, also when used in systems with more than 4 adsorbers, increases the internal pressure recovery and as a result, the product recovery efficiency, the offgas production is interrupted during said first stage of repressurizing, requiring large surge drums to dampen the irregularities of the offgas flow. Furthermore, an increase of the internal pressure recovery may have a limited effect on the product recovery efficiency in such cases, because for such increase, the following parameters are likewise increased: (1) the start-of-dump pressure, (2) the quantity of dump gas, (3) the content of the product component in the dump gas, and therefore (4) product loss. In addition, any such breakthrough of impurities will be at its maximum at said increased start-of-dump pressure and since the recovered secondary product gas is used for countercurrent low level repressurization of an adsorber, these impurities, due to the shortened range until the highest pressure is reached, will become re-adsorbed at a final position more close to the product end of said adsorber. Moreover, due to said shortened range, the magnitude of said re-adsorption will be less, leaving more of said impurities in the gaseous phase and therefore in the final product. Another way of increasing the internal pressure recovery efficiency and therefore of reusing a lower complementary proportion of secondary product gas for purging, is by increasing the number of participating adsorbers in the process like as described in the aforementioned patent to Batta, using 5 adsorbers and as described in U.S. Pat. No. 3,986,849 to A. Fuderer and E. Rudelsdorfer, using up to 10 adsorbers in the system. Each time when an adsorber is depresurrized by the cocurrent release of secondary product gas, such gas is distributed over more than one adsorber until for each next adsorber receiving such gas at a lower pressure level, equilibration is attained. In addition, the last portion of the released secondary product gas could be reused for low level repressurization instead of for purging as aforementioned for the Batta invention, however with the adverse effects of offgas flow interruption and of the increased start-of-dump pressure.
Still another invention is described in U.S. Pat. No. 4,350,500 to A. J. Esselink on the improvement of the internal pressure recovery efficiency, as is explained hereafter.
Because product gas is of the highest available pressure and of the highest purity, final repressurization takes place by the product split-off only, hence by high level repressurization. However where the flow rate of the net product should be kept constant, said product split-off should be withdrawn therefrom without interruption, leading to a surplus thereof when said product split-off is not needed for final high level repressurization. In cases where no useful purpose for said surplus other than for repressurization can be considered, said surplus is combined with said secondary product gas which is used for partial low level repressurization until pressure equilibration between two adsorbers, resulting into a higher equilibration pressure than if only secondary product gas had been used until such pressure equilibration. Contrary to the above and in accordance with the aforementioned Esselink invention, combining said product split-off and said secondary product gas is avoided in systems where at least two adsorbers are or will be in the condition of receiving feed gas and where by delaying the moment of switching on a next adsorber to adsorption, the final high level repressurization of this adsorber is likewise made to continue to be finally repressurized by said product split-off in a last stage, thereby eliminating said surplus. Since said product split-off no longer interferes with the pressure equilibration between two adsorbers, the equilibration pressure between said two adsorbers will be lower, causing the remaining part of secondary product to be used for purging to be lower as well. The number of adsorbers L, receiving feed gas in parallel will be at least two, whereby said number is temporarily reduced to Lxe2x88x921 after switching off one of these while the switching on of a regenerated adsorber to adsorption is delayed. During said delay said regenerated adsorber remains to be subjected to high level repressurization while consequently the velocity of the feed gas in the adsorber or adsorbers being switched on but as such in number been reduced by one, is temporarily increased by the ratio L/(Lxe2x88x921), provided the total stream of feed gas remains unchanged; the consequence of using this method of reducing the quantity of purge gas is, that the feed gas velocity in an adsorber for a fixed quantity of feed gas per adsorber varies in accordance with said ratio. Said variance is permitted as explained hereafter.
During adsorption, adsorption and desorption fronts are established inside the adsorbers concerned, marking the extent by which adsorbable components in the gaseous phase are carried along cocurrently with feed gas. Behind said fronts are active zones, known as mass transfer zones, where exchange of adsorbable components between the gaseous and adsorbed phase take place. The heights of these mass transfer zones depend (1), on diffusion parameters, affecting the resistance to mass transfer with respect to the exchange of adsorbable components between the gaseous and adsorbed phase, (2), on the driving forces for adsorption and desorption, and (3), on the feed gas velocity. Increasing or decreasing the feed gas velocity has a corresponding effect on the height of the mass transfer zones.
To achieve an efficient utilization of the adsorbent, the heights of the mass transfer zones should be small at the moment in time when adsorption and desorption fronts are no longer advancing, which is at the end of the cocurrent depressurization. Well ahead of said moment, the velocity of the cocurrent gas flow should be kept small, such that the heights of the mass transfer zones are allowed sufficient time to become correspondingly small towards said moment, as if no prior periods of higher gas velocities had existed. Because gas velocities during cocurrent depressurization will be almost zero near to the closed feed inlet end, the period for lower gas velocities affecting adsorption and desorption fronts, if more near to said feed inlet end, should preferably be chosen well ahead of the start of the cocurrent depressurization and therefore while still in the adsorption stage.
Therefore, without effect on the final adsorber conditions with respect to adsorbent loadings and adsorbate distribution between gaseous and solid phases, the adsorber feed gas velocity may be higher during a certain interval of time, provided said velocity is reduced again to a lower level, in general well ahead of the end of the adsorption step.
Surprisingly, it has been found that the internal pressure recovery efficiency can be significantly improved by implementing, during a part of said delay, an additional pressure equilibration, between said regenerated adsorbed and said switched off adsorber while the latter releases secondary product gas in a first stage, supplementary to the product split-off for repressurizing said regenerated adsorber. The high level repressurization is thereby divided into two stages, as such being considered a part of this invention. By practicing this invention under circumstances where the increase of the internal pressure recovery is beneficial, a significant improvement of the product recovery efficiency of 2% to 3% becomes feasible.